This invention relates generally as indicated to a centrifugal pump especially suited for use in pumping aircraft fuels and the like wherein the incoming fuel mixtures have a high ratio of fuel vapor and/or air and liquid fuel, termed V/L herein, and the fuel is delivered as a liquid fuel under pressure.
Aircraft fuels normally contain a certain amount of dissolved air which evolves when the pressure in the fuel tanks is lowered as may occur with increasing aircraft altitudes. The fuel is generally supplied to the engines of the aircraft by an engine driven, positive displacement pump, and in most cases the fuel tanks are far removed from and located well below the engine driven pump, whereby the fuel flow from the tank to the engine driven pump encounters additional pressure drop due to line resistance and change in height from the tank to the engine driven pump, producing relatively low pressure at the pump inlet. This low pressure at the pump inlet results in separation of the air dissolved in the fuel and partial evaporation of the fuel, creating a mixture of vapor and liquid. Also, the varying environment in which the aircraft operates may cause the fuel in the tanks to vary in temperature over a wide range from -65.degree. F. to +135.degree. F. or more, and higher fuel temperatures along with low fuel pressures result in additional fuel vaporization, which makes it difficult to deliver liquid fuel from the tanks to the aircraft engines at the required flow rates and pressure. The pump must recompress the incoming vapor and also add energy to the incoming fluid to deliver all liquid fuel at the pressure required by the fuel system components downstream of the pump.
Heretofore, various pump designs have been used for handling incoming fuels with a vapor to liquid ratio of up to 0.45, but when the V/L ratio exceeds 0.45, most pumps cannot operate without unacceptable loss in the delivered flow and pressure. Typical of the types of pumps which have been successfully used to pump fuels with up to 0.45 V/L ratio are centrifugal pumps where a portion of the vapor is returned back to the fuel tank; inducer type centrifugal impellers and jet pumps which have relatively good vapor handling capabilities; and positive displacement pumps with large bypass flow under normal operation used to make up for the incoming vapor. As the V/L ratio of the incoming fuels increases, however, these various pumps start to cavitate, resulting in a substantial drop in the delivered flow and/or pressure performance of the pumps below desired operating levels.
Cavitation of the engine driven pumps can be avoided as by pressurizing the fuel tanks or by utilizing a tank mounted boost pump to provide the desired fuel pressure at the inlet to the engine driven pump. However, in this arrangement, the pressure of the fuel in the line from the tank to the engine driven pump is higher than ambient, which is undesirable since if the line from the tank to the engine driven pump is punctured or ruptured in any way, the fuel will leak from the line creating a serious fire hazard.
Such higher pressure in the line from the tank to the engine driven pump can be kept to a minimum by mounting the boost pump closer to the engine driven pump, but that requires that the boost pump be capable of operation with a lower inlet pressure or higher V/L ratio of the fuel entering the boost pump than in the case of the tank mounted condition due to the additional pressure loss created in the line from the tank as a result of frictional losses and change in height from the tank.